All original Cherry keycaps in Cherry-profile made out of
double-shot moulded ABS are thick-walled.
Cherry has made (and is still making) keyboards with keycaps in Cherry-profile in PBT with laser-etched legends that are thin-walled, but with the notable exception of the stabilised keys which are still thick-walled.
Interference with backwards-facing MX switches is really an issue only for the shallowest row: the home row ... which on a standard keyboard will have a horizontally stabilised Enter key only if the layout is ANSI.
So, yes, on an ISO keyboard with backwards-facing switches you would not get interference if you use these original Cherry-profile keycaps.
But these laser-etched PBT keys are some kind of beige and/or grey, look kind'a cheap and are not very much desirable anyway unless your priority is to get the layout for a specific country/language. Plus, there may be other compatibility problems such as with the non-centered stem on the Caps Lock and Space Bar keycaps.
Cherry did also make all thick-walled PBT keycaps for a period in the '90s: some with laser-etched letters and some (more desirable) with dye-sublimated legends.
All of those are winkeyless (Edit:
Apparently wrong), but not all winkeyless vintage Cherry keyboards have thick-walled keycaps.
Beware though that Cherry PBT keys in Cherry profile also exists on Cherry keyboards with MY switches or rubber domes. Non-stabilised keys on MY keyboards will fit Cherry MX and may be thin-walled ... but the stems on 1.25 modifier keys are not in the centre. The rubber dome keyboards have completely different stems.
Now, I'm also assuming that you are not planning on using QMX clips, in which case only standard switch orientation and thick-walled Cherry/GMK keycaps work. (I think even EnjoyPBT has interference with these)
Edit: The ThermalTake Poseidon does definitely not have Cherry-made, or even Cherry-profile keycaps. The person must have referred to them
fitting Cherry switches.
That said, Cherry has been an OEM for other brands in the past, but ... that was the past, with older keyboard models.